Two ideas come to my mind. One would be to have them search pictures of
objects in magazines or internet. Compile what students think are similar and
actually go about measuring the copies! You will probably need to discuss
scale factors.
You can also take an object, trace it on the board, use an overhead to
enlarge the shape, trace new shapes and have students measure and explain why
the shapes are similar. You can use all different kinds of things with the
overhead!
-Claudia, for the Teacher2Teacher service